Timed constant on from a momentary switch?

Firstly, let me say this is a pretty cool place, and I wish I wasn't just wondering on, asking for advice, and wondering off. My specialisation is 2 wheelers and engineering, though, not electronics, and whilst I have a better understanding than the general public, I quickly get lost when long words are banded about so I wouldn't be much of an asset!

What I'm looking at doing is to annoy anyone attempting to nick my motorbike. The basic principle will be a hidden switch, somewhere, that controls whether the "start" button* activates the horn for ~30 seconds (using a separate battery so the bike will still start when I get back!) or starts the bike. I found a thread here which is a similar project to mine, but doesn't have the 30 second stipulation, and so will run the battery flat... great! On mine, the plan is the horn can be disabled either with another hidden switch (If I/a friend activates it accidentally) or will turn off automatically after 30 seconds, but can then be re-activated immediately if the scummy thieves are still mad enough to be trying their luck. Any suggestions? And please avoid using long words

*Would it be a fair bet that this start button is carrying a very small current to a relay, rather than carrying the full brunt of the power for the starter motor?

Thanks for the responses- the 555 is so close to perfect it's a joke! Unfortunately the input will normally be 0V, with the trigger at +12V, rather than 0V. The switch is handlebar mounted, and so would be pretty tricky to change.

I've thought about an inverting op-amp, but they can only go as low as the supply voltage will go, and it's pretty tricky to get a voltage of -12V from batteries....

Aha! I think I've got it! Simply wire up a relay or a mosfet so the trigger switch closes when the 12V gets to the mosfet, thus making the 555 timer do it's thing to another relay, eventually reaching the horn and annoying everyone in a mile radius . Sound like this'll work?

It can work but note that if the trigger switched is left pressed the alarm won't stop.

Click to expand...

The trigger is a push to make switch; as soon as pressure is removed it'll deactivate the mosfet, and then 20 to 30 seconds later the horn will (should?) stop annoying everyone, earlier if I hit my "oops" switch hidden away somewhere else and reset the circuit.

That said, it's good to know about the possible quirks. Testing this is going to be... "interesting" . Find the most abandoned, idyllic countryside I can and completely ruin it for a couple of minutes..... Apologies in advance to any walkers!!

And, if everything does go horribly wrong, one assumes ripping wiring out is the same with 20 Amps as it is with 2 Amps

Belt and braces. Think of it this way: They lift the bike into their van. Get it to their garage, break the steering lock, ignition, smash off the disc lock (yes I'll have one of those as well ) and then hit the start button. If I ever find my bike gone one day, at least I can rest secure in the knowledge that I very likely made some thief require a change of undies and a new set of eardrums .

As for tilt, vibration and similar sensors, the problem with those is that sometimes a bike will need to be moved legitimately- i was in a transit camping for a weekend, and a biker parked such that we were blocked in; with the help of a couple of guys I lifted the back end of the bike and swung it round so we could get out. If someone goes for the start button, then you KNOW they are up to no good, whereas there can be legitimate reasons for moving the bike slightly. If there were a way to easily set up a warning system, then I'd be all for it, but I suspect that's slightly beyond my ability

I've been thinking about this, and I think it'd be wiser to rig it to the ignition barrel as the starter is too easy to bypass on a motorbike. Is there any way to make it so that if the initial input stays active (i.e, chav sticks screwdriver in ignition, turns it, horn goes off, chav runs leaving screwdriver in place), the alarm will still deactivate after a certain time period?